This invention relates to a process for producing alumina from bauxite and, more particularly, to a Bayer process with an improved red mud separation stage.
In the Bayer process for producing alumina from bauxite, the bauxite containing aluminum trihydroxides or aluminum oxide-hydroxides is contacted with solutions containing caustic soda to dissolve the aluminum hydroxides as sodium aluminate while leaving most of the remaining constituents of the bauxite essentially unattacked in solid form. A part or all of the silica content of the bauxite may also dissolve in the caustic soda solution to form a soluble sodium silicate. This reacts relatively slowly with the sodium aluminate in solution to form complex hydrated sodium aluminum silicates, known collectively as "desilication product". These desilication products are of low solubility in the resulting sodium aluminate-caustic soda solutions and largely precipitate out of solution thereby removing much of the undesirable silica from the solution phase. However, there is the substantial cost of an appreciable loss of chemically-bound caustic soda and alumina in the desilication product.
After the digestion step for dissolving the aluminum hydroxide from the bauxite, the undissolved part of the bauxite, together with any desilication product that has precipitated at this point, which are known as "red mud", are separated from the solution, usually by filtration or sedimentation or both. The red mud is then disposed of, usually after being washed to recover the soluble valuables from the entrained caustic-aluminate solution. The clear caustic-aluminate solution after precipitation of the red mud, commonly known as "pregnant liquor", is subsequently cooled, diluted, seeded with aluminum trihyroxide crystals (gibbsite) and agitated for a period of time to precipitate a significant fraction of the dissolved alumina as gibbsite. This precipitate is then separated from the resulting spent liquor, which typically still contains in the order of half of the original dissolved alumina. A part of the separated gibbsite may be recirculated as seed material to the aluminum precipitation operation, while the remainder is washed to recover the soluble valuables from the entrained liquor, and is then suitably calcined to for alumina product of the Bayer process. The spent liquor may be re-concentrated, impurities removed and new caustic soda added as caustic feed to the digestion step.
The solubility characteristics of the aluminum hydroxides and caustic soda solutions require that the digestion step be carried out at high caustic soda concentration and high temperature in the circuit, and that the gibbsite precipitation step be carried out at low caustic concentration and low temperature in circuit. The levels of caustic soda concentration and temperature are typically determined by the type of aluminum hydroxide present in the bauxite, process economics and equipment constraints.
Key parts of the Bayer process consist of the digestion step and the mud separation step in which the aluminum hydroxide minerals of the bauxite are brought into solution in caustic-aluminate solution as soluble sodium aluminate and the remaining insoluble residue (red mud) is separated from the resulting pregnant solution, leaving a clear caustic soda-sodium aluminate solution from which purified gibbsite can subsequently be crystallized. Since the nature of the solubility of the aluminum hydroxide minerals in caustic soda solutions usually requires that the digestion step be carried out at an elevated temperature in order to achieve higher solubilities of the alumina and hence reasonable liquor productivity (weight of alumina produced per volume of liquor circulated), while the precipitation step needs to be carried out at much lower temperatures to minimize the alumina solubility at this point in the process, it can be seen that equipment must be provided for heating the incoming liquor and bauxite to the temperature required for digestion and for cooling the liquor and red mud solids after digestion.
Most current Bayer plants make use of a digestion and mud separation module consisting basically of the equipment required to carry out the following sequence of operations:
(1) Preheating the incoming spent caustic aluminate liquor and bauxite passing to the digesters, using as much as possible recuperated heat followed by high-temperature heat from an external source; PA1 (2) Carrying out the digestion while usually providing a residence time sufficient to permit removal of most of the silica dissolved from clay or quartz minerals in the bauxite by precipitation of a complex sodium aluminosilicate desilication product; PA1 (3) Cooling the digested slurry by flashing the slurry at one or more decreasing pressures down to about atmospheric boiling temperature and using the flashed steam recovered for preheating purposes; PA1 (4) At or below atmospheric boiling temperature, separating the red mud residue from the pregnant aluminate liquor, typically by filtration, or by flocculation, sedimentation and polish-filtration of the clear solution.